Wetlands: Sources or Sinks of Greenhouse Gases

Wetlands are an interface between terrestrial and aquatic ecosystems. They are often termed as 'nature's kidneys', because they cleanse our environment. In wetlands, soil is saturated with moisture either permanently or for a period long enough to support aquatic plants. It is an ecosystem whose formation, processes and characteristics are determined largely by water. Wetlands are a very sensitive ecosystem that are fully dependent on the hydrological conditions. They can be affected by both water stress as well as water surplus conditions. There is a definite hydrological condition, peculiar to each wetland type. The inflow and outflow of water should be in balanced form for a healthy wetland. But nowadays, this balance is under threat, primarily due to global warming and thus, wetlands are under severe pressure. Sometimes, they get heavy precipitation resulting in flooding, while sometimes they suffer from an acute drought condition, in any case leading to their degradation. A degraded wetland could pose grave danger to the environment as explained later. In ecosystems, the transportation and transformation of chemicals is known as biogeochemical cycling. Wetlands, owing to their diverse hydrological conditions, influence these cycles prominently. There is a definite chemical mass balance in wetlands. They act as a source of chemicals to the atmosphere, as a sink or "holder" for chemicals from the atmosphere and transforms them from one form into another.

Wetlands are usually known for mass carbon storage and sequestration in form of biomass, however they are also a prominent source of GHGs (Greenhouse Gases, e.g. $N_2O$, $CH_4$ etc.). This dual nature of wetlands is a very peculiar property and not so well understood, even though it is very clear that under certain circumstances same wetlands could either be net sink or net source of GHGs. The circumstances are usually anthropogenic, i.e. improper wetland management, ignorance of wetlands etc., which lead to their degradation.

Plants in wetland have a high growth rate, so they capture (or sequester) large amounts of carbon dioxide ($CO_2$). Furthermore, their soils are largely anaerobic so carbon that gets incorporated into the soil decomposes very slowly and can persist for hundreds or even thousands of years (carbon storage). Thus under these circumstances, wetlands are aptly considered a GHG sink. Now, let us see how they could act as GHG source. There are two prominent GHGs, methane and nitrous oxide which are natural products of wetlands. Wetland also store a much huge amount of methane: methane is currently 2nd most important anthropogenic GHG. Methane possesses 25 times higher global warming potential (GWP) per molecule than $CO_2$ over a time period of 100 years. Wetlands are single largest natural $CH_4$ source with a median emission of 164 Tg yr-1. Furthermore, temperature response of $CH_4$ production is much higher than respiration or photosynthesis i.e. its productions increases with rise in temperature, thus constituting a positive feedback system. Wetlands are place where excess Nitrogen got fixed by process of nitrification. The source of this excess nitrogen could be fertilizers from farmlands, plant biomass nitrogen (proteins etc.) or direct nascent nitrogen from atmosphere. Nitrogen fixation is processed in aerobic zone of wetlands but at the same time, in anaerobic zone, a complementary process also occurs, which is known as denitrification where nitrite transforms to Nitrous oxide. This Nitrous oxide when reaches to atmosphere starts behaving as GHG. The GWP per molecule of Nitrous Oxide is 298 times than $CO_2$ over 100 year time periods. Thus, the dual nature of wetlands are evident in terms of GHG sink and source. But, it is should not be inferred that wetlands are culprits of Global Warming. The main culprits are humans. The natural balance in wetlands is such that the net GHG productions and annihilation is nearly balanced. Problems occur when this critical balance is disturbed and the existence of wetlands is threatened.

There are many factors threatening the existence of wetlands. Some of the major ones which are usually caused by human actions are agriculture, encroachments, mining, dumping, global warming, dam constructions, draining, and development of springs etc. The anthropogenic impacts which are identified by IWRB (International Waterfowl and Wetlands Research Bureau) are, loss of wetland areas, changes in water regime, changes in water quality, unsustainable exploitation of wetland products and introduction of new species. We are encroaching the wetlands, filling them up, making townships and colonies over them. We are disturbing this very critical and important ecosystem in every manner. By threatening their existence,inadvertently, we are threatening our own existence. It seems a need of this time that we start caring about the nature and natural phenomena. We need to see the climate change as a consequence our own deeds. Incorporation of wetland management practices with climate change studies should be done in India. As of now, in India, very few wetland management studies addressing climate change exist. There should be a protocol encapsulating the climate change scenario in the management procedures for all other wetlands in India. With 1.25 Billion population, it is really taxing to implement something suddenly, but we should understand that everything needs beginning.